1. Field of the invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer having a fixing device.
2. Description of related art
It is known that a film heating technique is used for a fixing device arranged in an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer. Such a fixing device includes a heater that has an energized heat generating resistive layer on a ceramic substrate, a film that moves while being in contact with the heater, and a pressure roller that comes into contact with the film to form a nip portion.
A recording material bearing an unfixed toner image is heated while being pinched and conveyed by the nip portion of the fixing device. In this way, the toner image is fixed onto the recording material. This type of the fixing device requires a shorter period of time from when energization of the heater is started to when a temperature reaches the fixable temperature. Namely, this type of the fixing device is advantageous in on-demand capability.
Thus, a printer including this fixing device can output the first image more quickly after receiving a print command. In addition, as another advantage, this type of the fixing device requires less power consumption during a standby state while waiting for a print command.
However, this type of the fixing device has a problem with a temperature increase at a non-sheet-passing portion. More specifically, if the fixing device consecutively passes recording materials having a smaller width (hereinafter referred to as small-size recording materials) than that of maximum apparatus-conveyable recording materials (hereinafter referred to as maximum-size recording materials) in a direction orthogonal to a recording material conveyance direction, the temperatures at the non-sheet-passing portions are increased.
If recording materials of various sizes (widths) can pass through the fixing area, a fixing area through which recording materials pass will be referred to as a sheet-passing area and fixing areas other than the sheet-passing area will be referred to as non-sheet-passing areas. In addition, a surface of a heating member such as a film or a pressure member such as a pressure roller that passes through the sheet-passing area during rotation will be referred to as a sheet-passing-area passing surface. In addition, surfaces of the heating member that pass through the non-sheet-passing areas during rotation will be referred to as non-sheet-passing-area passing surfaces.
When the fixing device passes and fixes a maximum-size recording material, the surface of the heating member exhibits an approximately even temperature distribution in the entire fixing area. However, when the fixing device consecutively passes and fixes small-size recording materials, the surface temperature in a non-sheet-passing area of the heating member is excessively increased. This is because, if the fixing device consecutively passes small-size recording materials, no heat is removed by the recording materials in the non-sheet-passing area through which the small-size recording materials do not pass. As a result, the heat is partially accumulated.
Generally, under a condition where more heat is taken by recording materials, this temperature increase becomes more significant at the non-sheet-passing part. For example, the temperature increase becomes more significant if more recording materials are processed per unit time (higher productivity), if the grammage of the recording materials is large, or if the recording materials are used in a low-temperature environment where the recording materials are cooled.
If the fixing device consecutively passes small-size recording materials and the temperature increase is caused at the non-sheet-passing part, for example, supporting members of the heating member or a heating device are used at a temperature over heatproof temperatures of the supporting members. As a result, durability life of the apparatus is shortened.
Japanese Patent Application Laid-Open No. 2007-187816 discusses a method for controlling such temperature increases at the non-sheet-passing parts. According to this method, cooling fans and the like are arranged as a cooling unit to directly cool the heated non-sheet-passing parts of a heating member. In addition, according to this method, temperature detection units are arranged at the non-sheet-passing areas of the heating device or the heating member. In this way, by actively supplying cool air to the non-sheet-passing areas in amounts according to the temperature detected at the non-sheet-passing areas, the temperature increase at the non-sheet-passing part can be controlled. In addition, according to this method, by changing the cooled area according to the recording material width, recording materials having different widths can be handled.
There are cases where the width-direction center of a recording material pinched at the nip portion of the fixing device is conveyed with a displacement (hereinafter referred to as a positional displacement) of about 1 to 5 mm from a conveyance reference in a direction orthogonal to the recording material conveyance direction of the image forming apparatus.
For example, a cause of this positional displacement is a dimensional variation of a regulation member that comes into contact with an end of a recording material in a sheet cassette and that regulates movement of the recording material in the direction orthogonal to the recording material conveyance direction.
In addition, if a conveyance member for conveying a recording material to the fixing device has a variation in conveyance capability in the direction orthogonal to the recording material conveyance direction, a positional displacement could be caused. In addition, a positional displacement could be caused depending on the way a user loads a recording material on a sheet cassette. If such positional displacement of a recording material is caused, one of the non-sheet-passing areas is increased relative to the other non-sheet-passing area.
If a non-sheet-passing area is increased caused by a positional displacement, a larger amount of heat is accumulated in the non-sheet-passing area per unit time, compared with a case where no positional displacement exists or a case where a sufficiently small positional displacement exists. Namely, the temperature at the non-sheet-passing part is increased more quickly.
As discussed in Japanese Patent Application Laid-Open No. 2007-187816, there is an image forming apparatus having an air supply unit capable of changing the air supply area for cooling a non-sheet-passing area that varies depending on the recording material size (width). However, the cooling capability of such an image forming apparatus is set assuming that no positional displacement exists. Thus, if a positional displacement is caused, the cooling capability cannot accommodate the speed of the temperature increase at the non-sheet-passing part. As a result, the temperature increase at the non-sheet-passing portion is temporarily worsened.
If a large fan having a greater cooling capability is used in view of a positional displacement, the size of the apparatus is increased, which is problematic. Even if a large fan having a greater cooling capability is used, a larger amount of heat is still accumulated in the non-sheet-passing area of the pressure roller or the like until the fan is driven, compared with a case where no positional displacement exists or a sufficiently small positional displacement exists.
In this case, part of the heat accumulated in the non-sheet-passing area is transferred to a recording material, and an excessive amount of heat is supplied to the toner. As a result, a defective image is formed by a high-temperature offset or the like, which is problematic.
A conceivable solution is to suppose a situation in advance where a positional displacement is to be caused and to drive a cooling fan before the temperature increase at the non-sheet-passing part becomes significant. However, if no positional displacement exists, the non-sheet-passing area is excessively cooled. Consequently, the amount of heat to be supplied to the toner is reduced by the cooling fan, resulting in defective heating. Therefore, a defective image could be formed by a low-temperature offset or the like.